HYDROGEN STORAGE ON GRAPHENE SHEET: PHYSISORPTION, DIFFUSION AND CHEMISORBED PATHWAYS BY FIRST PRINCIPLES CALCULATIONS

Hydrogen is frequently touted as the "fuel of the future" because of its huge potential as clean energy source, although the large-scale adoption of this technology has yet to be realized. One of the remaining barriers to the utilization of hydrogen energy is an efficient and inexpensive means of hydrogen storage. In this work we investigate the nature of this process by first principle calculation. In particular, we study the way in which the H-2 molecule can interact with graphene sheet through physisorption and chemisorption mechanism. The first mechanism involves the condensation of the hydrogen molecule on the graphene as a result of weak van der Waals forces, while the chemisorption mechanism involves the preliminary dissociation of the H-2 molecule and the subsequent reaction of hydrogen atoms with the unsatured C-C bonds to form C-H bonds. To study carefully the possible physisorbed configurations on the graphene sheet, we take in to account van der Waals (vdW) interactions in DFT using the new method (DFT/vdW-WF) recently developed in our group and based on the concept of maximally localized Wannier functions. There are three possible way in which the H-2 molecule can adapt to the structure of graphene: the hollow, the bridge and the top site called H, B and T configurations, respectively. We find the hollow site to be most stable physisorbed state with a binding energy of -50 meV. This value, in agreement with experimental results, is also compared with other vdW-correction methods as described in the following paper. Diffusion of the physisorbed configurations on the graphene sheet and activated reaction pathways in which the molecule starts from a physisorbed configuration to end up in a chemisorbed configurations have also been studied.